1. Field of the Invention
This invention relates to LED arrays for use in an exposure light source (printer head) for an electrophotographic printer, and more particularly to highly integrated LED arrays with a density or resolution higher than 1200 DPI (Dot Per Inch).
2. Description of Related Art
Japanese Utility Model Preliminary Publication (KOKAI) No. 62-60053 discloses one such conventional LED array as a light source for use in an electrophotographic printer. FIGS. 6A-6B illustrate the construction of the conventional LED array 4, FIG. 6A being a top view of the LED array 4 and FIG. 6B being a cross-sectional view taken along lines 6B--6B of FIG. 6A.
As shown in FIGS. 6A-6B, the LED array 4 includes a plurality of LEDs 40 aligned in a row on an n-type semiconductor substrate 41. Each of the LEDs 40 includes a first interlayer dielectric 42a, first window 46 formed in the first interlayer dielectric 42a, second interlayer dielectric 42b, second window 48 formed in the second interlayer dielectric 42b, p-type diffusion region 43, p-electrode 44, and n-electrode 45.
The second window 48 is in alignment with the first window 46 and is of the same size as or slightly larger than the first window 46. In other words, the first window 46 is within the second window 48. The p-type diffusion region 43 is formed in the n-type semiconductor substrate immediately below the first window 46. The p-electrode 44 extends into the first window 46 and contacts the p-type diffusion region 43. The n-electrode 45 is common to all of the LEDs 40 and is formed on the reverse side of the n-type semiconductor substrate 41.
The p-type diffusion region 43 and n-type semiconductor substrate 41 constitute a pn junction 51 which emits light when energized. The p-electrode 44 is formed to cover the p-type diffusion region 43 both at a part 47a of the diffusion region surface 47 and at a part 46a of the first window 46. When a current flows between the p-electrode 44 and the n-electrode 45, the pn junction 51 emits light which emanates from a light-emitting area 47b not covered with the p-electrode 44.
Using the first interlayer dielectric 42a as a selective diffusion mask, zinc which is a p-type impurity is diffused from the first window 46 into the n-type semiconductor substrate 41, forming the diffusion region 43. The second interlayer dielectric 42b is formed in order to ensure the insulation between the p-electrode 44 and the n-type semiconductor substrate 41 even when pin holes are developed in the first interlayer dielectric 42a. Forming the second interlayer dielectric 42b on the first interlayer dielectric 42a improves the yield of the LED array 4.
With the aforementioned conventional LED array 4, if the LED array 4 is to have a ultra-high density higher than 1200 DPI, then the first windows 46 must be very small and arranged at very small intervals accordingly. Correspondingly, the second windows 48 must also be very small. Therefore, the mask must be registered with high accuracy during the photolithography process for forming the second windows 48 regardless of whether the second windows 48 are of the same size as or larger than the first windows 460. If the first windows 46 are partly outside of the second windows 48 due to poor alignment accuracy, the p-electrode 44 may have a smaller area in contact with the p-type diffusion region 43, that is, the area of the part 46a decreases. A decrease in the area of the part 46a increases contact resistance, resulting in poor characteristics and therefore lower yield of the LED arrays.